Tony--thanks for the guidance. I can see now that I've been wasting some of my camera's capabilities by only shooting at either ISO 100 or 800 and not using the ISOs in between.
JEH--The idea of a different IS0 "sweet spot" for each sensor makes intuitive sense given that we live in a world ruled by the laws of physics.
But when I buy a new camera, how do I determine that "sweet spot"? Can I glean that information somewhere in the many excellent charts and graphs published on this site? Or is like the olden days of film when one did a lot of testing with different ISOs and development times to determine how to get the best out of each film batch.

As we saw in an earlier blog post, the ISO definition used when you take photographs is based on the brightness of the resulting JPEGs, not on any inherent property of the sensor. Which is something to remember whenever you see anyone talking about manufacturers ‘cheating’ on their ISO figures. ...

Could someone explain how I can use this information to pull better performance out of my D200? All I really want is gorgeous midtones with better transitions between levels, especially at the shoulders of the curve. Nice skin tones, good rendition of subtle textures, that kind of thing. As we say in the States, 'what's the take away'?

As we saw in an earlier blog post, the ISO definition used when you take photographs is based on the brightness of the resulting JPEGs, not on any inherent property of the sensor. Which is something to remember whenever you see anyone talking about manufacturers ‘cheating’ on their ISO figures. ...